One type of non-volatile memory known in the art relies on magnetic memory cells. These devices, known as magnetic random access memory (MRAM) devices, include an array of magnetic memory cells. The magnetic memory cells may be of different types. For example, a magnetic tunnel junction (MTJ) memory cell or a giant magnetoresistive (GMR) memory cell.
Generally, the magnetic memory cell includes a layer of magnetic film in which the magnetization is alterable and a layer of magnetic film in which the magnetization may be fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization may be referred to as a sense layer or data storage layer and the magnetic film that is fixed may be referred to as a reference layer or pinned layer.
Conductive traces (commonly referred to as word lines and bit lines or collectively referred to as write lines) are routed across the array of memory cells. Word lines extend along rows of the memory cells and bit lines extend along columns of the memory cells. A memory cell stores the bit of information as an orientation of magnetization at each intersection of a word line and a bit line. The orientation of magnetization in the sense layer aligns along an axis of the sense layer that is commonly referred to as its easy axis. Magnetic fields are applied to flip the orientation of magnetization in the sense layer along its easy axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer.
The orientation of magnetization of each memory cell will assume one of two stable orientations at any given time. These two stable orientations, parallel and anti-parallel, represent logical values of “1” and “0”. The orientation of magnetization of a selected memory cell may be changed by supplying current to a word line and a bit line crossing the selected memory cell. The currents create magnetic fields that, when combined, switch the orientation of magnetization of the selected memory cell from parallel to anti-parallel or vice versa.
The resistance of the memory cell differs according to the parallel or anti-parallel orientation of magnetization. When the orientation is anti-parallel, i.e., the logic “0” state, the resistance of the memory cell is at its highest. The resistance of the memory cell is at its lowest when the orientation is parallel, i.e., the logic “1” state. As a consequence, the logic state of the data bit stored in the memory cell can be determined by measuring its resistance.
In one configuration, conductive traces (commonly referred to as sense conductors) are routed across the array of memory cells. These sense conductors extend along rows of the memory cells and are electrically coupled to the reference layers of the memory cells. The bit lines, which extend along columns of the memory cells, are electrically coupled to the sense layers of the memory cells. A memory cell is situated at each intersection of a sense conductor and a bit line.
In operation, a read circuit for sensing the resistance of a memory cell is electrically coupled to each sense conductor and bit line. The read circuit selects one sense conductor and one bit line to determine the resistance and state of a particular memory cell. In one configuration, the read circuit supplies a sense current that flows through the bit line and memory cell stack to sense conductor, and back to the read circuit, where a voltage is detected. This voltage is used to determine the resistance and state of memory cell.
A write circuit for writing the state of each memory cell is electrically coupled to each word line and bit line. During a write operation, the write circuit selects one word line and one bit line to set the orientation of magnetization in the sense layer of the memory cell at the cross point of the selected bit line and word line. The orientation of magnetization in the sense layer of the selected memory cell is rotated in response to currents on the selected bit line and word line. These currents generate magnetic fields according to the right hand rule, which act in combination to rotate the orientation of magnetization in the sense layer. A larger current in a write line produces a stronger magnetic field around the write line. This magnetic field drops off in strength with increasing distance from the write line.
The magnetic field present at the sense layer is a strong function of the spacing between the sense layer and the write lines. The greater the distance between the sense layer and the write line, the larger the current must be to maintain the same magnetic field strength in the sense layer. However, larger currents and resulting stronger magnetic fields may affect the state of adjacent memory cells in the array of memory cells. Additionally, larger currents may cause electro-migration problems in the write lines, and larger currents also require using bigger drive transistors, which consume valuable space on the magnetic memory device. Larger currents and stronger magnetic fields are not always a viable option for increasing magnetic field strength to write a memory cell.